Turbochargers are a type of forced induction system. They deliver air, at greater density than would be possible in the normally aspirated configuration, to the engine intake, allowing more fuel to be combusted, thus boosting the engine's horsepower without significantly increasing engine weight. A smaller turbocharged engine, replacing a normally aspirated engine of a larger physical size, will reduce the mass and can reduce the aerodynamic frontal area of the vehicle.
Turbochargers use the exhaust flow from the engine exhaust manifold to drive a turbine wheel (21), which is located in the turbine housing (2). Once the exhaust gas has passed through the turbine wheel and the turbine wheel has extracted energy from the exhaust gas, the spent exhaust gas exits the turbine housing and is ducted to the vehicle downpipe and usually to after-treatment devices such as catalytic converters, particulate traps, and NOx traps.
In a wastegated turbocharger, the turbine volute is fluidly connected to the turbine exducer by a bypass duct. Flow through the bypass duct is controlled by a wastegate valve (61). Because the inlet of the bypass duct is on the inlet side of the volute, which is upstream of the turbine wheel, and the outlet of the bypass duct is on the exducer side of the volute, which is downstream of the turbine wheel, flow through the bypass duct, when in the bypass mode, bypasses the turbine wheel, thus not powering the turbine wheel. To operate the wastegate, an actuating or control force must be transmitted from outside the turbine housing, through the turbine housing, to the wastegate valve inside the turbine housing. A wastegate pivot shaft extends through the turbine housing. Outside the turbine housing an actuator (73) is connected to a wastegate arm (62) via a linkage (74), and the wastegate arm (62) is connected to the wastegate pivot shaft (63). Inside the turbine housing, the pivot shaft (63) is connected to the wastegate valve (61). Actuating force from the actuator is translated into rotation of the pivot shaft (63), moving the wastegate valve (61) inside of the turbine housing. The wastegate pivot shaft rotates in a cylindrical bushing (68), or directly contacts the turbine housing. Because an annular clearance exists between the shaft and the bore of the bushing, in which it is located, an escape of hot, toxic exhaust gas and soot from the pressurized turbine housing is possible through this clearance.
Turbine housings experience great temperature flux. The outside of the turbine housing faces ambient air temperature while the volute surfaces contact exhaust gases ranging from 740° C. to 1050° C. depending on the fuel used in the engine. It is essential that the actuator, via the translated motions described above, be able to control the wastegate to thereby control flow to the turbine wheel in an accurate, repeatable, non jamming manner.
A VTG is used not only to control the flow of exhaust gas to the turbine wheel but also to control the turbine back pressure required to drive EGR exhaust gas, against a pressure gradient, into the compressor system to be re-admitted into the combustion chamber. The back pressure within the turbine system can be in the region of up to 500 kPa. This high pressure inside the turbine stage can result in the escape of exhaust gas to the atmosphere through any apertures or gaps. Passage of exhaust gas through these apertures is usually accompanied by black soot residue on the exit side of the gas escape path. This soot deposit is unwanted from a cosmetic standpoint, and the escape of said exhaust gas containing CO, CO2, and other toxic chemicals can be a health hazard to the occupants of the vehicle. This makes exhaust leaks a particularly sensitive concern in vehicles such as ambulances and buses. From an emissions standpoint, the gases which escape from the turbine stage are not captured and treated by the engine/vehicle aftertreatment systems.
Typically, some of the leakage of gas and soot through the annulus formed by a shaft rotating within a cylindrical bore was tolerated since the end faces of the bushing are usually in contact with either the inboard flange of the valve arm or the outboard flange or surface of the driving arm of the wastegate control mechanism, thus blocking leakage some of the time.
Seal means such as seal rings, sometimes also called piston rings, are commonly used within a turbocharger to create a seal between the static bearing housing and the dynamic rotating assembly (i.e., turbine wheel, compressor wheel, and shaft) to control the passage of oil and gas from the bearing housing to both compressor and turbine stages and vice versa. BorgWarner has had seal rings for this purpose in production since at least 1954 when the first turbochargers were mass produced. For a shaft with a seal ring boss of 19 mm diameter, rotating at 150,000 RPM, the relative rubbing speed between the seal ring cheek and the side wall of the seal ring groove is of the order of 149,225 mm/sec.
Seal rings, of the variety which are used as described above, are sometimes used as a sealing device for relatively slowly rotating shafts (as compared to the 150,000 RPM turbocharger rotating assembly seals). These slowly rotating shafts move in rotational speeds of the order of 15 RPM which equates to a relative rubbing speed of 7 to 8 mm/sec.
Seal rings, as used in turbochargers, create a seal by contacting part of the side wall of the seal ring against one side wall of the seal ring groove and contacting the outside diameter of the seal ring against the inside diameter of the bore in which the shaft resides. In order for the ring to be assembled to the shaft and then the shaft and ring be assembled into a bore, the depth of the seal ring groove must be such that the ring can collapse in outside diameter (and thus effective circumference and inside diameter) so that the outside diameter of the seal ring can assume approximately the inner diameter of the bore in which it operates. FIG. 2A depicts a seal ring (80) in the naturally expanded condition, albeit assembled to the shaft by forcibly expanding the ring over the diameter of the shaft (63) and then allowing the ring to relax into the groove. As the shaft, with the ring assembled on it, is pushed into the bore of the bushing (68), a chamfer (69) compresses the ring until the outside diameter of the ring can slide in the inside diameter (70) of the bushing. The now-compressed ring seals against the inside diameter of the bushing at any axial position of the shaft.
In this condition, as depicted in FIG. 3, the seal ring (80) can axially reside at any axial position within the confines of the ring groove, the seal ring groove being defined as: the volume between the radial elements of the outside diameter of the shaft (86) and the diameter of the floor (82) of the seal ring groove; and the distance between the inner (83) and outer (81) walls of the seal ring groove. With this definition of the seal ring groove, it can be seen that there always exists a volume under the ring, (ie between the inside diameter (84) of the compressed piston ring, and the diameter of the floor (82) of the seal ring groove. There also can exist a volume between the inner wall (83) of the seal ring groove and the proximate wall of the seal ring. On the opposite side of the seal ring groove, there can also exist a volume between the outer wall (81) of the seal ring groove and the proximate wall of the seal ring. FIG. 3 depicts a condition in which the seal ring (80) is somewhat centered between the inner and outer walls (83 and 81) of the seal ring groove, thus allowing passage of gas and soot (86) around the seal ring. Since the axial position of the seal ring is controlled by the friction between the inner diameter of the bore in the bushing, and the ring is only moved by any contact with a side wall of a groove, a nearly complete sealing condition only exists when the seal ring sidewall is in direct contact with a seal ring groove side wall. In any other axial condition, the leakage path depicted in FIG. 3 exists.
Various arrangements of seal rings are known, each arrangement operating in a slightly different manner. In the case of a single seal ring as shown in FIG. 3, the fluid (e.g., exhaust gas) will flow from higher pressure to lower pressure. A significant pressure drop would occur across the seal ring. To improve the effectiveness of the seal, it is known to use two or more of such seal rings in sequence, each ring typically seated in it's own groove. In such a case, as the pressure moves in one direction across the seals, there would be a pressure drop across each seal ring, and the effect would be cumulative. In addition to such “passive” seals, “active” seals are also known. These introduce a slight level of pressure or vacuum into the space between two rings, thereby interrupting the flow of exhaust gas across the sequential seals. However, such an “active seal” system requires bores and/or piping to connect the space between the two ring seals with a source of pressure or vacuum. Further, only the slight pressure or vacuum necessary to achieve the desired “flow interruption” effect is used. The problem of exhaust gas flow past the individual ring seals is not addressed or solved by this system.
It would be advantageous to have a seal system that was improved over the known systems. It would also be advantageous to be able to retrofit a turbocharger with an improved seal system without requiring extensive modification of the turbocharger.
Thus it can be seen that there is a need for a design to produce a complete gas seal for, e.g., wastegate and VTG pivot shafts in turbochargers.